Country doctor that's injected one guy from the south (7)
I believe the answer is:
vietnam
'country' is the definition.
(I know that Vietnam is a sovereign state)
'doctor that's injected one guy from the south' is the wordplay.
'doctor' becomes 'vet' (vet is a kind of doctor).
'that's injected' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'guy' becomes 'man ' (guy can informally mean a man).
'from the south' is a reversal indicator (in a down clue, letters go up).
'man' in reverse letter order is 'nam'.
'vet' enclosing 'i' is 'viet'.
'viet'+'nam'='VIETNAM'
(Other definitions for vietnam that I've seen before include "Country of south-eastern Asia" , "Asian country, divided north and south from 1954 to 1975" , "Country at war with itself from the 1950s to 1970s" , "East Asian country" , "Far East country" .)